In view of the current situation of unclear understanding of the genesis of the low resistivity oil layer in Guantao Formation of CFD6-4 oilfield in Bohai Sea, the microscopic and macroscopic genetic mechanism of the low resistivity oil layer was systematically analyzed by using clay mineral analysis, scanning electron microscope, particle size analysis, heavy mineral analysis, core nuclear magnetic resonance and other data combined with the study of sedimentary evolution. The research shows that the low resistivity oil layer is rich in clay minerals compared with the conventional resistivity oil layer. The illite mixed layer and illite layer are bridged to fill the pores to form a conductive network. The clay minerals distributed in the porous network fully contact with the formation water to produce cation exchange, forming the microgenesis of the low resistivity oil layer. The complex pore structure leads to high capillary bound water porosity, which also leads to lower oil saturation in low resistivity reservoirs, which constitutes another cause of formation of low resistivity reservoirs. Low-resistance oil reservoirs in the study area are mainly developed at the end of the half-cycle of the rise of the medium-term base level. On the whole, the river energy is weak, and the sand-carrying capacity is reduced. For example, the fine-grained sediment of clay minerals is gradually enriched, and the increase of the proportion of fine-grained sediment causes the complexity of the pore structure of the reservoir, and the increase of the bound water saturation, which constitutes the macro cause of the development of low-resistance oil reservoirs.